1. Field of the Invention
The present invention relates to vapor compression systems such as refrigeration, air-conditioning, heat pump systems and/or a combination of these, operating under transcritical or sub-critical conditions using any refrigerant and in particular carbon dioxide, and more specifically but not limited to an apparatus operating as a reversible refrigeration/heat pump system.
2. Description of Prior Art
A non-reversible vapor compression system in its basic form is composed of one main circuit which provides a compressor 1, a heat rejecter 2, a heat absorber 3 and an expansion device 6 as shown in FIG. 1. The said system can function either in heating or cooling mode. To make the system reversible, i.e. to enable it to work as both heat pump and refrigeration system, known prior arts use different system design changes and addition of new components to the said circuit to achieve this goal. The known prior arts and their disadvantages are now described.
The most commonly used system comprises a compressor, a flow reversing valve, an interior heat exchanger, an internal heat exchanger, two throttling valves, two check valves, exterior heat exchanger and a low-pressure receiver/accumulator, see FIG. 2. The reversing is carried out using the flow reversing valve, two check valves and two throttling valves. The disadvantage of this solution is that it uses two throttling valves and the fact that the internal heat exchanger will be in parallel flow in either heating or cooling mode, which is not favorable. In addition, the solution is little flexible and can not be effectively used with systems using an intermediate-pressure receiver.
EP 0604417 B1 and WO90/07683 disclose a transcritical vapor compression cycle device and methods for regulating its supercritical high-side pressure. The disclosed system includes a compressor, gas cooler (condenser) a counter-flow internal heat exchanger, an evaporator and a receiver/accumulator. High-pressure control is achieved by varying the refrigerant inventory of the receiver/accumulator. A throttling device between the high-pressure outlet of the counter-flow internal heat exchanger and evaporator inlet is applied as steering means. This solution can be used either in heat pump or refrigeration mode.
Additionally DE19806654, describes a reversible heat pump system for motor vehicles powered by an internal combustion engine where the engine coolant system is used as heat source. The disclosed system uses an intermediate pressure receiver with bottom-feed flashing of high pressure refrigerant in heat pump operation mode that is not ideal.
Further, DE19813674C1 discloses a reversible heat pump system for automotive air conditioning where exhaust gas from the engine is used as heat source. The disadvantage of this system is the possibility of oil decomposition in the exhaust gas heat recovery heat exchanger (when not in use) as the temperature of the exhaust gas is relatively high.
Still further, U.S. Pat. No. 5,890,370 discloses a single-stage reversible transcritical vapor compression system using one reversing device and a special made reversible throttling valve that can operate in both flow directions. The main disadvantage of the system is the complex control strategy that is required by the special made throttling valve. In addition, in its present status, it can only be applied to single stage systems.
Yet another patent, U.S. Pat. No. 5,473,906, disclosed an air conditioner for vehicle where the system uses two or more reversing devices for reversing system operation from heating to cooling mode. In addition, the patented system has two interior heat exchangers. Compared to the present invention, in one of the proposed embodiment of the said patent, the arrangement is such that the interior heat exchanger is placed between the throttling valve and the second reversing device. The main disadvantage of this arrangement is that the low-pressure vapor from the outlet of the interior heat exchanger has to pass through the second reversing device which results in extra pressure drop for the low-pressure refrigerant (suction gas) in cooling mode. In heating mode, the system suffers also from a higher pressure drop on the heat rejection side of the system as the discharge gas has to pass through two reversing devices before it is cooled down. In another embodiment from the said patent, the same interior is placed between the first reversing device and the compressor. This embodiment again results in a higher pressure drop on the heat rejection side in heating mode operation. In yet another embodiment, the compressor is in direct communication with said two four way valves. Again this embodiment results in extra pressure drop for the low-pressure refrigerant (suction gas) in cooling mode as the said suction gas has to pass through the said two four way valves before entering the compressor. In heating mode, it also suffers from a higher pressure drop. In addition, the placement of the receiver after the condenser in the proposed embodiments is such that it can only be used for conventional system with condenser and evaporator heat exchanger and as such it is not suitable for transcritical operation since the devised pressure receiver does not have any function in transcritical operation. Another general drawback of the system is that the patent does not provide embodiments for other application such as simple unitary system, two-staged compression, combined water heating and cooling as the present invention does since the said patent was intended exclusively for vehicle air conditioning.
Regarding the second aspect of the present invention, US-Re030433 refers to condenser and evaporator operation of the heat exchanger, while the present application is concerned with evaporator and gas cooler operation. In the latter case, refrigerant is a single-phase fluid, and condenser draining is not an issue. In a gas cooler, the purpose is often to heat the air flow over a range of temperature, and this cannot be done if the sections of the heat exchanger operate in parallel on the air side. Thus, in gas coolers, the design of the circuit will be different than in a heat exchanger that needs to serve as a condenser. In the present application, air always flows sequentially through the sections of the heat exchanger, while in the US-Re030433 invention, air flows through all “heat transfer zones” in parallel.
Another patent, US-Re030745 discloses a reversible heat exchanger which has many similarities to the one above (US-Re030433), including the fact that operation is limited to evaporator and condenser modes. Also in this case, the air flows in parallel through all sections. Another important difference is that the patent describes a heat exchanger where all sections are connected in parallel on the refrigerant side during evaporator operation. In the present application, the refrigerant usually flows sequentially through the heat exchanger also in evaporator mode.
In essence, the present application describes a reversible heat exchanger that serves as a heater in one mode—by cooling supercritially pressurized refrigerant and heating air—while it operates as an evaporator in another mode, in both cases the refrigerant and the air flows sequentially through the sections. The only difference is that in gas cooler operation refrigerant flows sequentially through all sections in counterflow with the air, while in evaporator operation, two and two sections are connected in parallel.
These aspects are not covered by these two said patents, and neither of the above patents would serve the desired purposes in gas cooler operation.